legally if you accept ANYTHING (even free drink coupons) in exchange for your singing which puts you in a position of 'working for them' then free speech doesn't enter into it - as your right of free speech doesn't extend to when you are representing a corporate entity.

Likewise - though you are 'in public' at a Borders - it is NOT a public space - but a private one - and they are again within their right whether or not you are working for them to restrict what you say and or do.